The ink-jet printing process involves the ejection of fine droplets of an ink composition onto a print medium such as paper in response to electric signals generated by a controller, such as a microprocessor. The fine droplets are typically emitted by a printing head that passes over the print medium without contacting the print medium.
The printing head, in some embodiments, may include an orifice plate that has very small nozzles through which the ink droplets are ejected. The nozzles can be, for instance, less than 50 microns in diameter. The nozzles are in fluid communication with the ink composition. Ink drop ejection is currently achieved either thermally or piezoelectrically. In thermal ink-jet printing, each nozzle is associated with a resistor element. Each resistor element is in turn connected to a microprocessor, whose signals direct one or more resistor elements to heat up rapidly. This causes a rapid expansion of ink that forces a drop of ink through the associated nozzle onto the print medium.
In piezoelectric ink-jet printing, ink droplets are ejected due to the vibrations of piezoelectric crystals stimulated by electrical signals generated by a microprocessor.
Many ink-jet ink compositions contain a dye dissolved in a solvent. The dye is typically anionic. For instance, sulfonated dyes are widely used in ink-jet applications. Of particular advantage, these dyes may be dissolved or dispersed in an aqueous-based system.
Unfortunately, however, many ink-jet inks are susceptible to dye aggregation. Dye aggregation may be especially prominent when using higher molecular weight dyes, such as those having molecular weights approaching 1,000 g/mol. Dye aggregation can also occur if the dye composition contains background salts and other impurities which may be left over fragments from commercial dye synthesis. The formation of dye aggregates can cause the dye to coalesce and form much larger clusters which lead to nozzle fouling. Once nozzle fouling occurs, the precision of the ink-jet printer becomes compromised. In fact, clogging of the nozzle can lead to the complete breakdown of the printer.
As such, a need currently exists for a solubility additive which inhibits dyes from coalescing in dye compositions and therefore leads to higher dye solubility.
In particular, a need exists for an additive that hinders the flocculation of dye aggregates into macro-particles in order to prevent the nozzles of a printing head from fouling.